The present invention relates to a screw fastening structure and a nut member, and in particular, to a screw fastening structure of the type that includes a male thread and a female thread to be electrically connected to each other, and a nut member of the type that has a female thread constituting a circuit element to be electrically connected to a mating male thread.
In application to a grounding of a vehicle body, such a type of screw fastening structure as well as such a type of nut member tends to suffer an insulating paint coat that covers a male thread, thus intervening between the male thread and a mating female thread.
As a countermeasure, there has been proposed a paint removing groove defining a thread profile as an edge for removing a paint layer, as disclosed in Japanese Utility Model Publication No. 57-147410.
However, in the field of vehicle in which the painting is always improved, the provision of a paint removing groove sometimes fails to insure a neat removal of an intervening paint layer, resulting in remaining pieces of paint as electrical insulator pressed between male and female threads screwed under strong thrust. Such paint pieces may cause a non-conformity in current conduction between the screwed threads.
The present invention has been achieved with such points in view.
It therefore is an object of the invention to provide a screw fastening structure and a nut member, permitting a female thread to be electrically connected to a mating male thread covered with an insulating paint coat, with a secured current conduction through the paint coat.
To achieve the object, an aspect of the invention provides a screw fastening structure comprising a conductive male thread having a first flank covered with an insulating paint coat, a conductive female thread having a second flank, and an electric contact interconnecting the first and second flanks through the paint coat.
According to this aspect, the second flank of the conductive female thread is electrically connected to the first flank of the conductive male flank, by the electric contact interconnecting them through the paint coat, permitting a secured current conduction.
The electric contact may preferably comprise a conductive projection formed on the second flank.
Further to achieve the object, another aspect of the invention provides a nut member comprising a conductive female thread having a flank, and a conductive projection formed on the flank.
Preferably, the nut member may further comprise a substantially straight nut body formed with the female thread and provided with a polarity identifier for identifying a polarity in orientation of the projection.